1. Field of the Invention
The present invention relates to an image fiber for use in industrial and medical endoscopes and general image transmission systems, as well as a fabrication process thereof. The present invention also relates to an image fiber preform appropriate for use in the image fiber, as well as a fabrication process for the preform.
2. Prior Art
It is well known that an optical fiber strand including a SiO.sub.2 -glass core having a high refractive index and a SiO.sub.2 -glass cladding containing fluorine (F) and/or boron oxide (B.sub.2 O.sub.3) for reducing the refractive index of the cladding may be used in order to obtain the necessary large refractive index difference between the core refractive index and the cladding refractive index, whereby the diameter of the optical fiber may be reduced. For example, when the core diameter is 4 .mu.m and the cladding outer diameter is 6 .mu.m, the relative refractive index difference between the core refractive index and the cladding refractive index needs to be about 4%. A plurality of the above mentioned optical fiber strands, which are arranged closely in a glass tube, are fused by heating from an edge thereof and drawn to form an image fiber.
An example of the fused-type image fibers is shown in FIG. 5. The image fiber 21 is composed of cores 22, a common cladding part 23 disposed between these cores 22, and a jacket 24 which surrounds the common cladding part 23. The image fiber 21 can transmit pictures from one end-face of an objective side to the other end-face of an ocular side through multiple cores 22. The refractive index distribution on the A-part of the cross-section of the image fiber shown in FIG. 5 is indicated in FIG. 6. In general, the refractive index distribution n(r) is calculated by the following equation (1): EQU n(r)=n.sub.0 {1-2.DELTA.(r/a).sup..alpha. }.sup.1/2 ( 1)
wherein n.sub.0 is the refractive index of a center of a core; r is the distance from the center of the core to a random point in the core; a is the radius of the core; .DELTA. is the relative refractive index difference (.DELTA.=(n.sub.0 -n.sub.c)/n.sub.0 wherein n.sub.0 is the refractive index of the center of the core and n.sub.c is the refractive index of a cladding); and .alpha. is a refractive index distribution constant. For the refractive index distribution, it is known that a step-type refractive index distribution in which the refractive index is constant has a refractive index distribution constant (.alpha.) of 5 and more and that a graded-type refractive index distribution in which the refractive index is not constant but parabolically changes has a refractive index distribution constant (.alpha.) of about 2.
The image fiber having a refractive index distribution shown in FIG. 6 is composed of cores 22, each core having a graded-type refractive index distribution and a common cladding part 23 having a flat refractive index distribution. The graded-type refractive index distribution of the core 22 has a refractive index distribution constant (.alpha.) of about 2. However, since the cladding-glass doped fluorine (F) and/or boron oxide (B.sub.2 O.sub.3) has a lower softening temperature than the pure silica glass has in this type of image fiber, the cladding glass is dissolved when a plurality of optical fiber strands are arranged and fused, thus causing the array of cores to disarrange. In the case of the cladding-glass doped with fluorine (F), the surface of the cladding is subjected to etching by the fluorine to generate silicon tetrafluoride (SiF.sub.4) gas during the melting process. The SiF.sub.4 gas forms bubbles which result in decreasing the mechanical strength of the image fiber.
In order to prevent the array of cores from disarranging, it is proposed that an image fiber strands further include support layers disposed at the outer peripheral surface of the claddings are arranged closely in a glass tube and subsequently fused to obtain an image fiber. The support layer, which is a pure silica layer composed of silicon(IV) oxide (SiO.sub.2) without dopant, has a higher softening temperature than the cladding has.
An example of the fused-type image fibers mentioned above is shown in FIG. 7. The image fiber 25 is composed of cores 26, cladding parts 27, each cladding part 27 surrounding each core 26, a high refractive index support layer 28 disposed between these cladding parts 27 surrounding the cores 26 and a jacket 29 which surrounds the high refractive index support layer 28. FIG. 8 shows a refractive index distribution on the C-part of the cross-section of the image fiber shown in FIG. 7. Both the core 26 and the high refractive index support layer 8 have respectively step-type refractive index distribution, which has a refractive index distribution constant (.alpha.) of about 5 and more.
FIG. 9 shows another example of refractive index distribution of an image fiber having the same structure as the fiber shown in FIG. 7. This image fiber is composed of cores 30, each core 30 having a graded-type refractive index distribution, cladding parts 31, each cladding part 31 surrounding each core 30, and a high refractive index support layer 32 formed between these cladding parts 31 surrounding the cores 30, the support layer 32 having a step-type refractive index distribution.
These image fibers having the refractive index distribution shown in FIGS. 7 and 8 have an advantage that the array of cores is not disarranged. However, in the image fibers, disadvantageously bubbles are generated at the boundary face where the cladding and the high refractive index support layer contact, thus causing to decrease the mechanical strength of the image fibers since the composition of the cladding is extremely different from that of the high refractive index support layer. Furthermore, as the image fibers have step-type high refractive index support layers, unwanted light such as excess incident light and the like adversely diffuse into the core, thus causing deterioration of the contrast of the image fiber.